Indusium

Indusium, a luxuriant growth from the leaf in some ferns (q.v.), covering more or less completely the sorus or cluster of sporangia. In Nephrodium it is a kidney-shaped epidermal structure, but it may be several cells thick, and even have stomata.

In Hymenophylleae it is a marginal cup-shaped structure, and in Lygodiese a similar one envelops each separate sporangium. In many species of Pteris and other genera a false indusium is produced by the rolling back of the margin of the leaf.